Screw assembly for cooking extruders operating with significantly reduced specific mechanical energy inputs

ABSTRACT

Screw assemblies are provided for cooking extruders characterized by operation using significantly reduced specific mechanical energy inputs, wherein the screw assemblies include first and second elongated, axially rotatable screws which are substantially coextensive in length, with the screw flighting presenting a pair of axially spaced apart sections of short pitch length and an intermediate section of greater pitch length. The intermediate section has an axial length greater than about four times the length of the axially spaced apart sections. The screw assemblies permit incorporation of significantly greater amounts of steam into comestible products during cooking thereof, as compared with prior art designs.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 12/980,018 filed Dec. 28,2010, now abandoned, which is a continuation of Ser. No. 12/208,517filed Sep. 11, 2008, now abandoned, both of which are incorporatedhereby by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is broadly concerned with improved extruders andmethods for extrusion processing of comestible products with very lowspecific mechanical energy (SME) inputs as compared with conventionalmethods. More particularly, the invention is concerned with extrudershaving specially configured screws designed to permit addition of veryhigh quantities of steam, so that the amount of SME required to forcomplete cooking is maritally reduced. The resultant feeds have veryhigh cook values and expansion characteristics.

2. Description of the Prior Art

Extrusion processing of comestible products such as human foods andanimal feeds has long been practiced and is a highly developed art. Ingeneral terms, food extruders of the single or twin screw variety areemployed, having elongated, tubular barrels with inputs adjacent one endthereof and restricted orifice dies at the outlet thereof, and one ortwo helically flighted, rotatable screws within the barrel. In manyinstances steam is injected into the barrel during processing,conventionally by means of injectors oriented at a perpendicular anglerelative to the longitudinal axis of the barrel. Depending upon theselected extrusion conditions, the final products may be fully orpartially cooked, and can have varying degrees of expansion. Thisrequires a total energy input into the materials being processed, whichusually has two components, energy derived from steam injection(specific thermal energy, STE) and SME. It is well known in the art thatSME input is significantly more expensive than STE input (generally2-2.5 times more expensive), and accordingly reducing the extent of SMEinput required to produce a given product would be very attractive froman economic point of view.

Conventional extrusion systems are limited in the amount of steam whichcan be injected into the extruder barrel, typically no more than about5% by weight. This in turn means that SME input must be increased toprovide the necessary energy input required. Consequently, the extrusionequipment must have a more robust and therefore construction than wouldotherwise be necessary, the extent of extruder component wear is higherthan desirable, and utility costs are increased.

One factor influencing SME inputs in extrusion processes is the flightdepth ratio of the extruder screw(s). This is the ratio of the outerdiameter of the screw (S_(D)) to the root diameter of the screw (R_(D)).The flight depth ratio largely determines the shear and volume of outputfrom an extruder. Typical cooking extruders in use today have a flightdepth ratio in the range of 1.3 to 1.8. Skilled artisans understand thata ratio of 1.3 is too small, in that the screw(s) lack free volume,creates excessive shear inputs and consumes too much power. Similarly, aratio above 1.8 is deemed too large, in that the screw has too much freevolume and which will prevent barrel fill and the ability to achieve adesired cook value. Accordingly, a flight depth ratio of 1.5-1.6 isconsidered to be the best compromise. For example, the Wenger TX85twin-screw extruder has a flight depth ratio of 1.574.

Another geometrical consideration is the ratio of the pitch of theextrusion screw(s) to the flight depth ratio. Smaller values of thisratio for a given pitch translate into lower exit temperatures,increased mechanical efficiency (i.e., more product throughput for agiven power input), and greater outputs. The Wenger TX85 extruder with1.5 pitch screws has a pitch/S_(D)/R_(D) of 0.953, and with 0.5 pitchscrews, the value is 0.317.

It is also the general practice in the extrusion art to positionadjacent, intermeshed twin screws in a close, self-wiping orientation.Any significant axial gap or clearance between the adjacent flightingsections is considered to be detrimental in that it could create deadzones of accumulated product, and also would decrease the extent ofproduct mixing within the extruder barrel. For example, the Wenger TX85has an axial gap of 0.039 inches between the adjacent flighting, andthis is in keeping with the conventional wisdom of extruder design.

Generally, designers of extruders follow these guidelines in order toachieve what is thought to be the best compromise between extruder sizeand utility costs on the one hand, versus the need to provide fullycooked and expanded products on the other. That is, an extruder can bedesigned with smaller drives and screw geometries which will minimizeSME. However, these types of extruders will be deficient in that cookvalues will be unacceptably low and significant expansion cannot beachieved. Alternately, an aggressive extruder design can be used, whichwill assure adequate cook and expansion of products, but this willinevitably result in high shear and SME inputs with resultant highercosts.

There is accordingly a need in the art for improved extruder equipmentand methods which achieve the seemingly contradictory goals of lowcapital and utility costs with reduced SME inputs, while at the sametime being capable of producing fully cooked and expanded products ofhigh quality.

SUMMARY OF THE INVENTION

The present invention overcomes the problems outlined above and providesimproved, low-SME input extruders and corresponding methods for theproduction of fully cooked comestible extrudate products such as humanfoods or animal feeds. Broadly speaking, the invention provides a methodof extruding a material using a cooking extruder (preferably a twinscrew extruder) having an elongated, tubular barrel with an inlet and anoutlet and presenting a longitudinal axis, a restrictive orifice dieplate disposed across the outlet, and an elongated, axially rotatable,helically flighted screw within the barrel. The method comprises thesteps of passing a starting material into the barrel inlet and rotatingthe screw to convey the material along the length thereof toward andthrough the die plate while imparting an SME input into the material.During passage of the material through the barrel, at least one zone iscreated within the barrel having free volume not occupied by thematerial (i.e., there is a free volume within the zone). Steam isinjected into the free volume zone of the barrel at an oblique angle(preferably from about 30-60°) relative to the barrel longitudinal axis,and the injected steam is mixed with the material being processed. Theamount of injected steam is in excess of 6% by weight steam (morepreferably at a level from about 7-25% by weight), based upon the totaldry weight of the starting material taken as 100% by weight. As aconsequence of these high injected steam levels, the SME input issignificantly lessened, and is up to about 22 kWhr/T (more preferablyfrom about 5-22 kWhr/T). The extruded products are normally highlycooked, exhibiting a cook value of at least 75%, more preferably fromabout 75-98%.

In more particular aspects of the invention, substantially fully cookedcomestible products selected from the group consisting of pet feeds andaquatic feeds are fabricated using SME input levels of up to about 18kWhr/T (more preferably from about 10-18 kWhr/T) for pet feeds, and SMEinput levels of up to about 16 kWhr/T (more preferably from about 8-16kWhr/T) for aquatic feeds. The methods of the invention can becontrolled for giving desired expansions of the final products.Expansions of up to about 50%, more preferably from about 15-35% canreadily be achieved.

The invention allows extrusion equipment to be produced at lower cost,owing to the fact that acceptable products can be produced with lowerSME inputs. That is, the equipment may be made with smaller motors anddrives than would otherwise be necessary. The internal extrudercomponents such as barrels, linings, and screws are also subjected toless wear. Furthermore, utility costs are reduced in as much as energyinputs derived from steam are much less expensive than those from SME.

The preferred extruders of the invention are twin-screw extruderswherein the screw assemblies are of a special and unique design. Thescrew assemblies include first and second elongated, axially rotatablescrews each having an elongated shaft with outwardly extending helicalflighting along the length of each shaft and with the flighting of eachshaft being intermeshed with the flighting of the other shaft. Eachflighting presents a pair of axially spaced apart sections of shortpitch length, and an intermediate section between the short pitch lengthsections and having a pitch length greater than the pitch lengths ofeither of the short pitch length sections. Also, the flighting of theintermeshed intermediate sections has a very large axial gap distancetherebetween of from about 0.1-0.4 inches. This has been shown toenhance distributive mixing of the material being processed within theextruder barrel.

Other features of the preferred screw assemblies include the ratio ofthe pitch length of the intermediate, long pitch length section to thepitch lengths of either of the first and second short pitch lengthsections. This ratio should be from about 1-7. Additionally, the ratioof the pitch length of the intermediate section to the flight depthratio should be from about 0.4-0.9. These features, particularly incombination, allow for very high barrel steam injection without blowbackthrough the barrel inlet and with complete mixing of the injected steaminto the material being processed. Consequently, an increased proportionof the total energy input needed to fully cook and expand the extrudateis derived from steam rather than SME.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a cooking extruder in accordancewith the invention, equipped with obliquely oriented steam injectionports and injectors;

FIG. 2 is a front end view of the cooking extruder depicted in FIG. 1;

FIG. 3 is a vertical sectional view taken along line 3-3 of FIG. 2;

FIG. 4 is a vertical sectional view taken along line 4-4 of FIG. 1;

FIG. 5 is a schematic illustration of an orthogonal resolution of thelongitudinal axis of one of the extruder barrel injection ports,illustrating the resolution components;

FIG. 6 is a plan view of a pair of intermeshed extruder screws for usein the preferred twin screw extruder of the invention;

FIG. 7 is an enlarged, fragmentary view of portions of the screws ofFIG. 6, illustrating the pitches and clearances between sections of thescrews;

FIG. 8 is a somewhat schematic plan view of a preferred preconditionerfor use with the extruder of the invention;

FIG. 9 is a front elevational view of the preconditioner of FIG. 6;

FIG. 10 is a vertical sectional view of a twin screw extruder of adifferent configuration as compared with the extruder of FIGS. 1-4,having steam injection ports and injectors located along the intermeshedregion of the extruder screws and oriented perpendicularly relative tothe longitudinal axes of the extruder screws; and

FIG. 11 is a vertical sectional view taken along line of 11-11 of FIG.10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred Extruder

Turning now to the drawing, a cooking extruder 10 in accordance with theinvention includes an elongated, tubular, multiple-section barrel 12presenting juxtaposed, intercommunicated chambers or bores 14, 16, and apair of elongated, helically flighted, axially rotatable, juxtaposed,intercalated screws 18 and 20 within the bores 14, 16. The barrel 12includes an inlet 22 and a spaced outlet 24 which communicate with thebores 14, 16. A restricted orifice die 25 is positioned across outlet 24for extrusion purposes and to assist in maintaining pressure within thebarrel 12. Additionally, the drive ends 26 of the screws 18, 20 areoperably coupled with a drive assembly (not shown) for axially rotationof the screws 18, 20, which typically includes a drive motor and gearreduction assembly.

In more detail, the barrel 12 includes, from right to left in FIGS. 1and 3, a series of tubular sections connected end-to-end by conventionalbolts or other fasteners. Specifically, the barrel 12 has an inlet head28, a first short steam restriction head 30, a first steam injectionhead 32, a second short steam restriction head 34, a mid-barreladjustable valve assembly head 36, an adjustable steam outlet head 38, asecond steam injection head 40, and third short steam restriction head42. As illustrated, each of the heads 28-34 and 38-42 is equipped withendmost, radially enlarged connection flanges 28 a-34 a and 38 a-42 a,and all of the heads 28-42 have aligned through-bores whichcooperatively form the barrel bores 14 and 16. The head 36 likewise hasthrough bores mating with those of flanges 32 a and 38 a.

The heads 32 and 40 of barrel 12 are each equipped with two series ofsteam injection ports 44 or 46, wherein each of the ports houses anelongated steam injector 48 or 50. The two series of ports 44 in head 32are located so as to respectively communicate with the bores 14 and 16of the head (see FIG. 4). Similarly, the two series of ports 46 in head40 also respectively communicate with the bores 14 and 16 of this head.

Importantly, the ports 44 and 46 are oriented at oblique angles relativeto the longitudinal axes of the corresponding bores 14 and 16. Inpractice, the ports are oriented at an angle from about 30-85 degrees,more preferably from about 30-60 degrees and most preferably about 45degrees, relative to these axes. Moreover, the ports 44, 46 arepreferably oriented in a direction toward the outlet 24. Morespecifically, and referring to FIG. 5, it will be seen that eachrepresentative port 44 presents a longitudinal axis 52. If this axis 52is orthogonally resolved into components 54 and 56, the component 54extends in a direction toward outlet 24.

The mid-barrel adjustable valve assembly head 36 is of the typedescribed in U.S. patent application Ser. No. 11/279,379, filed Apr. 11,2006 and incorporated by reference herein. Briefly, the head 36 includesopposed, slidable, flow restriction components 58 and 60, which can beselectively adjusted toward and away from the central shafts of theextruder screws 18 and 20, so as to vary the restriction upon materialflow and thus increase pressure and shear within the extruder 10. On theother hand, the steam outlet head 38 has a steam outlet 62 with anadjustable cover 64 permitting selective escape of steam during thecourse of extrusion. In some instances, a vacuum device (not shown) canbe used in lieu of cover 64 for more effective withdrawal of steamand/or reduction in processing pressures.

Attention is next directed to FIGS. 3 and 6-7 which depict the preferredextruder screws 18 and 20. These screws are identical in configuration,are substantially coextensive in lenghth and of single flight design,and are of the co-rotating variety (i.e., the screws rotate in the samerotational direction). It will be seen that each of the screws 18, 20broadly includes a central shaft 66 with helical flighting 68 projectingoutwardly from the shaft 66. The screws 18, 20 are specially designedand have a number of novel features. These features are best describedby a consideration of certain geometrical features of the screws andtheir relationship to each other and to the associated bores 14, 16. Inparticular, the shafts 66 have a root diameter R_(D) defined by thearrow 70 of FIG. 3, as well as an outermost screw diameter S_(D) definedby the screw flighting 68 and illustrated by arrow 72. In preferredpractice, the ratio S_(D)/R_(D) (the flight depth ratio) of the of theoutermost screw diameter to the root diameter is from about 1.9-2.5, andmost preferably about 2.35.

The individual sections of each screw flighting 68 also have differentpitch lengths along screws 18, 20, which are important for reasonsdescribed below. Additionally, along certain sections of the screws 18,20, there are different free volumes within the bores 14, 16, i.e., thetotal bore volume in a section of the barrel 12 less the volume occupiedby the screws within that section, differs along the length of thebarrel 12.

In greater detail, each screw 18, 20 includes an inlet feed section 74,a first short pitch length restriction section 76 within head 30, afirst longer pitch length section 78 within head 32, a second shortpitch length restriction section 80 within head 34, a second longerpitch length section 82 within heads 38 and 40, and a third short pitchlength restriction section 84 within head 42. It will thus be seen thatthe pitch lengths of screw flighting 68 of screw sections 76, 80, and 84are substantially smaller than the corresponding pitch lengths of theflighting 68 of the screw sections 78 and 82. In preferred practice, thepitch lengths of screw sections 76, 80, and 84 range from about 0.25-1.0screw diameters, and are most preferably about 0.33 screw diameters. Thepitch length of 78 and 82 range from about 1-2 screw diameters, and aremore preferably about 1.5 screw diameters. The ratio of the longer pitchlength to the shorter pitch length preferably ranges from about 1.5-7,more preferably from about 3-6, and most preferably about 4.5. As usedherein, “screw diameter” refers to the total diameter of a screwincluding the flighting thereof as illustrated in FIGS. 3 and 7.

The screws 18 and 20 also have very large flight depths as measured bysubtracting R_(D) from S_(D), and often expressed as the flight depthratio S_(D)/R_(D). This is particularly important in the long pitchsections 78 and 82, where the ratio of the pitch length to the flightdepth ratio (pitch length/S_(D)/R_(D) is from about 0.4-0.9, morepreferable from about 0.5-0.7, and most preferably about 0.638. In theshort pitch sections 76, 80 and 84, the ratio of the pitch length to theflight depth ratio is from about 0.1-0.4, more preferably from about0.15-0.3, and most preferably about 0.213.

The intermeshed longer pitch screw sections 78 and 82 of the screws 18,20 include a further unique feature, namely the very wide axial spacingor gap 86 between the respective screw sections. Preferably, this gap isfrom about 0.1-0.4 inches, more preferably from about 0.15-0.35 inches,and most preferably from about 0.236 inches. It should also be notedthat the corresponding axial spacing or gap 88 between the shorter pitchscrew sections 76 and 84 are much less, on the order of 0.039 inches.

These geometrical features are important in achieving the ends of theinvention, and specifically permit incorporation of significantlygreater amounts of steam into the material passing through extruder 10,as compared with conventional designs. Furthermore, the extruder iscapable of producing fully cooked, highly palatable comestible productshaving significantly reduced SEM inputs which materially reduce extrudercapital and utility costs. In essence, the restriction heads 30 and 34,and 34 and 42, together with the short pitch length screw section 76, 80and 84 therein, cooperatively create steam flow restriction zones whichinhibit the passage of injected steam past these zones. As such, thezones are a form of steam locks. Additionally, provision of the heads32, 38, and 40 with the longer pitch length screw sections 78 and 82therein, between the restriction zones, creates steam injection zonesallowing injection of greater quantities of steam than heretoforepossible. The longer pitch screw sections 78 and 82 result in decreasedbarrel fill (not necessarily greater free volume), and thus create steaminjection zones. An examination of the screws 18, 20 stopped undernormal processing conditions reveals that the screw sections 76 and 80are completely full of material, whereas the longer pitch screw sections78 and 82 are only partially full. The orientation of the injectionports 44 and 46, and the corresponding injectors 48 and 50 therein,further enhances the incorporation of steam into the material passingthrough extruder 10.

The longer pitch screw sections 78 and 82 generate excellent conveyanceof materials and incomplete fill of material, allowing for the unusuallyhigh level of steam injection. Moreover, the combination of the longerpitch lengths and very wide gap 86 create increased leakage flowresulting in gentle kneading of the moistened material within thesesections, particularly at relatively high screw speeds of up to 900 rpm.During wet mixing or kneading of steam and water into the material beingprocessed, low shear conditions are maintained, and the material canpass forwardly and rearwardly through the gap 86. At the same time, thegap 86 is small enough to create the desired distributive mixing ofsteam and water into the material.

This combination of factors within extruder 10 allows significantlygreater steam to be injected, as compared with conventional extruderdesign. In the later case, only about 3-5% steam may be injected, basedupon the total dry weight of the material being processed taken as 100%by weight. As used herein, “dry weight” refers to the weight of theingredient(s) making up the material without added water but includingingredient native water. Attempts to inject greater amounts of steam inconventional extruders normally results in the excess steam simplypassing backwardly through the extruder and exiting the barrel inlet.However, in the present invention, in excess of 6% by weight steam maybe successfully injected without undue injected steam loss, based upontotal weight of dry material within the barrel 12 at any instance takenas 100% by weight. More particularly, testing has shown that up to about15% by weight steam may be injected, but this limit is primarily basedupon steam injection capacities and not any limitations upon the abilityof the extruder to accept excess steam. Broadly therefore, the inventionpermits introduction of from about 7-25% by weight steam, morepreferably from about 10-18% by weight, and most preferably from about11-15% by weight.

It has been found that the extruder of the invention, by permittingincorporation of greater amounts of steam than heretofore believedpossible, permits cooking of food or feed products with materiallydecreased SME inputs. This in turn means that the extruder can be madeless expensively, with smaller motors and drive assemblies than wouldotherwise be required. Furthermore, utility costs are greatly reduced,because of the fact that energy in the form of steam is much lessexpensive than that derived from mechanical energy.

The invention is especially adapted for the low-SME production of feedsselected from the group consisting of pet feeds (e.g., dog and catfeeds) and aquatic feeds (e.g., floating, slow-sinking, and fast-sinkingfeeds for fish or other aquatic creatures). In this context, the petfeeds can be produced with an SME input of up to about 18 kWhr/T, andmore preferably at a level from about 10-18 kWhr/T. Likewise, theaquatic feeds can be produced with an SME input of up to about 16kWhr/T, more preferably from about 8-16 kWhr/T.

As more specific examples of the foregoing, the following table setsforth various classes of conventional extrudates and the SME inputsconventionally required to achieve a complete cook during processing.

TABLE 1 Typical Typical Range of SME Extruded Moisture Protein TypicalStarch Values Required Density Upon Levels Fat Level Level forProcessing Ranges Extrusion Product Category (% by wt) (% by wt) (% bywt) (kWhr/T) (g/l) (% by wt) Dog Food 18-26  4-10 25-45 20-28 380-41021-23 Cat Food 26-34  5-11 24-36 25-35 410-440 22-24 High Fat Pet Foods26-34 12-20 15-25 18-25 430-480 24-26 Floating Fish Feed 18-36 2-5 20-5020-25 410-460 21-23 Slow-Sinking Fish 26-45 20-40  5-15 25-40 510-57018-22 Feed Fast-Sinking Fish 26-48 18-26 10-15 18-25 600-650 26-28 FeedShrimp Feed 22-32 2-6 12-26 18-25 660-720 27-31

The products manufactured using the extruder of the invention normallyhave all of the same characteristics as products conventionallyextruded, but with SME inputs reduced by at least about 25%, morepreferably from about 25-50% below those of Table 1. Further, theproduct densities may be 5-10% lower than those set forth in Table 1,and as-extruded moistures may be lessened by 5-15%, if desired.

Thus, dog food products may be produced with SME inputs of from about8-21 kWhr/T, more preferably from about 10-18 kWhr/T; cat foods at SMEinputs of from about 12-27 kWhr/T, more preferably from about 14-20kWhr/T; high fat pet foods at SME inputs of from about 6-19 kWhr/T, morepreferably from about 10-16 kWhr/T; floating fish feeds at SME inputs offrom about 6-19 kWhr/T, more preferably from about 10-16 kWhr/T;slow-sinking fish feeds at SME inputs of from about 8-30 kWhr/T, morepreferably from about 12-21 kWhr/T; and fast-sinking fish feeds andshrimp feeds at SME inputs of from about 6-19 kWhr/T, more preferablyfrom about 10-16 kWhr/T. In terms of absolute values, the majority ofhuman food or animal feed products can be processed in accordance withthe present invention with SME inputs of up to about 22 kWhr/T, morepreferably from about 5-22 kWhr/T, and most preferably from about 9-16kWhr/T.

It well also be understood that the extent of expansion of a givenextrudate can greatly influence the amount of total energy inputrequired for production of the product. Thus, a highly expanded productof low density often requires a significantly greater total energy inputthan otherwise identical products having no or insignificant expansion.Therefore, the contribution of SME to the total energy input wouldusually be increased in highly expanded products, as compared withdenser products.

Although the extruder 10 illustrated in the Figures includes the use ofan adjustable valve assembly head 36 and steam outlet head 38, the useof such heads is not required. The head 36 can advantageously be used asa further restriction against steam loss, and the head 38 can be used ininstances where mid-barrel steam venting is desired, e.g., where denserproducts are desired. Further, although not shown, the extruder barrelmay be equipped with external jackets for introduction of heat exchangemedia to indirectly heat or cool the material passing through theextruders.

In the normal use of extruder 10 for cooking of comestible products suchas human foods or animal feeds, the following conditions are typical:residence time of the material being processed within the extruderbarrel of from about 3-20 seconds, more preferably from about 4-10seconds; extruder screw speeds of from about 250-900 rpm, morepreferably from about 400-800 rpm; maximum temperature of material beingprocessed within the barrel, 100-150° C., more preferably from about110-125° C.; maximum pressure within the barrel of from about 100-1000psi, more preferably from about 400-800 psi. In such processing thematerial may be cooked (as measured by extent of gelatinization ofstarch-bearing ingredients) to any desired level, but usually cooklevels of at least about 75%, more preferably from about 75-98% areachieved.

Preferred Preconditioner

Turning next to FIGS. 8-9, a preferred preconditioner 90 is depicted.This preconditioner is fully illustrated and described in US PatentPublication No. 2008/0094939, incorporated by reference herein. Broadly,the preconditioner 90 includes an elongated mixing vessel 92 with a pairof parallel, elongated, axially-extending mixing shafts 94 and 96 withinand extending along the length thereof. The shafts 94, 96 are operablycoupled with individual variable drive devices 98 and 100, the latter inturn connected with digital control device 102. The preconditioner 90 ispositioned upstream of extruder 10, such that the output from thepreconditioner is directed in to the outlet 22 of extruder barrel 12.

In more detail, the vessel 92 has an elongated, transversely arcuatesidewall 104 presenting a pair of elongated, juxtaposed,intercommunicated chambers 106 and 108, as well as a material inlet 110and a material outlet 112. The chamber 108 has a larger cross-sectionalarea than the adjacent chamber 106. The sidewall 104 has access doors114 and is also equipped with injection assemblies 116 for injection ofwater and/or steam into the confines of vessel 92 during use of thepreconditioner, and a vapor outlet 118. The opposed ends of vessel 92have end plates 120 and 122, as shown.

Each of the shafts 94, 96 extends the full length of the correspondingchambers 106, 108 along the center line thereof, and has a plurality ofradially outwardly extending paddle-type mixing elements (not shown)which are designed to agitate and mix material fed to thepreconditioner, and to convey the material from inlet 110 towards andout outlet 112. The mixing elements on each shaft 94, 96 are axiallyoffset relative to the elements on the adjacent shaft. Moreover, themixing elements are intercalated (i.e., the elements on shaft 94 extendinto the cylindrical operational envelope presented by shaft 94 and theelements thereon, and vice versa). The mixing elements may be orientedsubstantially perpendicularly to the shafts 94, 96. In otherembodiments, the mixing elements may be adjusted in both length andpitch, at the discretion of the user.

The drives 98 and 100 are in the illustrated embodiment identical interms of hardware, and each includes a drive motor 124, a gear reducer126, and coupling assembly 128 serving to interconnect the correspondinggear reducer 126 and motor 124 with a shaft 94 or 96. The drives 98 and100 also preferably have variable frequency drives 130 which aredesigned to permit selective, individual rotation of the shafts 94, 96in terms of speed and/or rotational direction independently of eachother. In order to provide appropriate control for the drives 98 and100, the drives 130 are each coupled between a corresponding motor 124and a control device 132. The control device 132 may be a controller,processor, application specific integrated circuit (ASIC), or any othertype of digital or analog device capable of executing logicalinstructions. The device may even be a personal or server computer suchas those manufactured and sold by Dell, Compaq, Gateway, or any othercomputer manufacturer, network computers running Windows NT, NovelNetware, Unix, or any other network operating system. The drives 130 maybe programmed as desired to achieve the ends of the invention, e.g.,they may be configured for different rotational speed ranges, rotationaldirections and power ratings.

In preferred forms, the preconditioner 90 is supported on a weighingdevice in the form of a plurality of load cells 134, which are alsooperatively coupled with control device 132. The use of load cells 134permits rapid, on-the-go variation in the retention time of materialpassing through vessel 92, as described in detail in U.S. Pat. No.6,465,029, incorporated by reference herein.

The use of the preferred variable frequency drive mechanisms 98, 100 andcontrol device 132 allow high-speed adjustments of the rotational speedsof the shafts 94, 96 to achieve desired preconditioning while avoidingany collisions between the intermeshed mixing elements supported on theshafts 94, 96. In general, the control device 132 and the coupled drives130 communicate with each drive motor 124 to control the shaft speeds.Additionally, the shafts 94, 96 can be rotated in different or the samerotational directions at the discretion of the operator. Generally, theshaft 94 is rotated at a speed greater than that of the shaft 96.

Retention times for material passing through preconditioner 90 can becontrolled manually by adjusting shaft speed and/or direction, or, morepreferably, automatically through control device 132. Weight informationfrom the load cells 134 is directed to control device 132, which in turnmakes shaft speed and/or directional changes based upon a desiredretention time.

The preconditioner 90 is commonly used for the processing of animal feedor human food materials, such as grains (e.g., wheat, corn, oats, soy),meat and meat by-products, and various additives (e.g., surfactants,vitamins, minerals, colorants). Where starch-bearing grains areprocessed, they are typically at least partially gelatinized duringpassage through the preconditioner. The preconditioner 10 is usuallyoperated at temperatures of from about 100-212° F., residence times offrom about 30 seconds-5 minutes, and at atmospheric or slightly abovepressures.

The drive arrangement for the preconditioner 90 has the capability ofrotating the shafts 94, 96 at infinitely variable speeds of up to about1,000 rpm, more preferably from about 200-900 rpm. Moreover, theoperational flexibility of operation inherent in the preconditionerdesign allows for greater levels of cook (i.e., starch gelatinization)as compared with similarly sized conventional preconditioners.

The following examples set forth the preferred apparatus and methods inaccordance with the invention. It is to be understood, however, thatthese examples are provided by way of illustration and nothing thereinshould be taken as a limitation upon the overall scope of the invention.

EXAMPLE 1

In these illustrative tests, data from a previous run using a WengerMagnum ST TX85 twin screw extruder was compared with the improved twinscrew extruder of the invention, in the preparation of aquatic feeds.The TX85 extruder is of the type illustrated in FIGS. 10-11, whereas theextruder of the invention is illustrated in FIGS. 1-5. As can be seen,both extruders include shorter pitch sections located on opposite sidesof longer pitch steam injection sections. In the case of the TX85, theinjectors were located in perpendicular relationship to the longitudinalaxes of the extruder screws, whereas in the improved extruder, theinjectors were oriented at a 45° angle relative to the screw axes.Moreover, the twin screws of the TX85 had a close flighting-to-flightinggap of about 0.039 inches along the entire lengths thereof, and thelonger pitch sections thereof had a significantly smaller flightingdepth (measured from the outer surface of the flighting to the shaft)and a consequently smaller S_(D)/R_(D).

In greater detail, the TX85 extruder 140 includes an elongated barrel142 made up of four interconnected heads, namely a first inlet head 144,a second steam injection head 146, third head 148, and fourth cone nosehead 150. The inlet head 144 has a material inlet 152, whereas theoutlet 154 of fourth head 150 is covered by a restricted die 156. A pairof intermeshed extruder screws 158, 160 extend along the length ofbarrel 142 and are connected to appropriate drive mechanism (not shown)for co-rotation of the screws during operation of extruder 140.Referring to FIG. 11, it will be observed that the screws 158, 160include, from inlet 152 to outlet 154, an inlet section, a short-pitchcompression section 164, a longer pitch steam injection section 166, asecond compression section 168, a variable pitch section 170, and afinal, tapered cone nose section 172. The compression sections 164 and168 cooperatively create a steam injection area 174 within barrel 142.Three steam injectors 176 are mounted on the barrel head 146 andcommunicate with area 174. These injectors 176 are oriented at aperpendicular angle relative to the longitudinal axes of thecorresponding screws 158, 160.

The upstream preconditioner used with the TX85 was a standard dual-shaftWenger DDC preconditioner of the type described in U.S. Pat. No.4,752,139. The shaft within the small diameter housing section wasrotated at a speed of 250 rpm, whereas the shaft within the largediameter housing section was rotated at 125 rpm.

The test (No. 1) using the TX85 extruder was carried out using a recipemade up of 40% by weight fish meal, 20% by weight soybean meal, 17% byweight corn gluten meal, 15% by weight wheat flour, and 8% by weightfull fat soy meal. The apparatus included a conventional Wenger CDCpreconditioner upstream of the inlet to the TX85. Further, an uprightback pressure valve (BPV) of the type illustrated in U.S. Pat. No.6,773,739 was located between the heads 148 and 150 of the extruderbarrel; this BPV was not functional in the process, however, owing tothe fact that it was maintained in an essentially full-open position andthus did not contribute any resistance to material flow or pressurebuildup within the barrel.

The other test (No. 2) using the improved extruder of the invention wascarried out using a recipe made up of 35% by weight wheat middlings, 17%by weight rice bran, 23% by weight wheat flour, 6% by weight fish meal,and 19% by weight full fat soybean meal. The apparatus included apreconditioner of the type depicted in FIGS. 8-9 hereof, with the smallcylinder shaft being operated in reverse (counterclockwise asillustrated in FIG. 9) at 800 rpm and the large cylinder shaft operatingforwardly (clockwise as illustrated in FIG. 9) at 50 rpm.

Although the equipment and some operational parameters used in tests 1and 2 are somewhat different, the important fact is that the energy andmoisture inputs in both tests are very similar, as are the retentiontimes. For example, the different preconditioner speeds gave a smalleffect compared with the overall process conditions employed. Moreover,the BPV was operated at a 99% open position, meaning that the effect ofthe valve was insignificant.

The following Table 2 sets forth the results from this series of tests.

TABLE 2 RUN NUMBER 1 2 DRY RECIPE INFORMATION Density (kg/m³) 557 608Feed Rate (kg/hr) 1991 2000 Feed Screw Speed (RPM) 40 40 PRECONDITIONINGINFORMATION Preconditioner Speed (RPM) 125/250 50/800 Steam Flow toPreconditioner (kg/hr) 180 160 ¹Steam Flow to Preconditioner (%) 9 8Water Flow to Preconditioner (kg/hr) 240 209 ¹Water Flow toPreconditioner (%) 12 14 Weight in Preconditioner (kg) 136 119Preconditioner Retention Time (min) 3 2.63 Preconditioner Discharge Temp(° C.) 77 73 Moisture Entering Extruder (% wb) 24.16 26.4 EXTRUSIONINFORMATION ²S_(D)/R_(D) 1.574 2.351 Size of Extruder Drive (kW) 187.5112.5 Extruder Shaft Speed (RPM) 600 425 Extruder Motor Load (%) 37 33.4Steam Flow to Extruder (kg/hr) 119 120 ¹Steam Flow to Extruder (%) 6 6Water Flow to Extruder (kg/hr) 40 0 ³Control/Temperature First Head (°C.) 40/44 — ³Control/Temperature Second Head (° C.) 70/83 —³Control/Temperature Third Head (° C.) 110/129 — ³Control/TemperatureFourth Head (° C.) 120/115 — Back Pressure Valve % Closed (%) 99 —Specific Mechanical Energy (SME) (kWhr/T) 29 12 Specific MechanicalEnergy (SME) (kJ/kg) 102.6 43.2 Steam Energy (STE) (kJ/kg) 424.8 396Total Energy Input (SME + STE) 527.4 439.2 ⁴Energy Costs ($/T) 4.96 3.67FINAL PRODUCT INFORMATION Extruder Discharge Moisture (% wb) 24.07 25.62Extruder Discharge Density (kg/m³) 441 435 ¹Percentage values define theamount of added moisture (water or steam) based upon the total dryweight of the ingredients taken as 100% by weight. ²S_(D)/R_(D) is theratio of the outside diameter of the screws to the root diameters of thescrews at the steam injection regions along the extruder barrel. This isa measure of screw volume. ³Temperature control involved injection ofcold water or steam into the external jackets of the extruder barrel.⁴Mechanical energy costs were assumed to be $18.06/million kJ, and steamenergy costs of $7.32/million kJ.

These tests were conducted so as to compare the energy costs at the samelevel of steam injection to the extruder. That is, the conventional TX85extruder could only be operated with a maximum 6% steam injection to theextruder. Accordingly, the improved extruder of the invention wasoperated at this same level of steam injection in order to compare thetotal energy costs. While the product densities were very similar inruns 1 and 2, the total energy costs for producing the respectiveproducts were significantly less in run 2.

EXAMPLE 2

In this set of tests, essentially the same extruder set-ups wereemployed, with no steam injection to the TX85 extruder, but with amaximum amount of steam injection to the improved extruder hereof. TheTX85 equipment did not include a BPV.

The recipe used with the TX85 (No. 3) was a proprietary pet food recipe,whereas the recipe in the improved extruder (No. 4) was the same as thatset forth in Example 1. The data collected in this set of runs is setforth in the following Table 3.

TABLE 3 RUN NUMBER 3 4 DRY RECIPE INFORMATION Density (kg/m³) 551 608Feed Rate (kg/hr) 1185 2000 Feed Screw Speed (RPM) 25 40 PRECONDITIONINGINFORMATION Preconditioner Speed (RPM) 125/250  50/800 Steam Flow toPreconditioner (kg/hr) 120 160 Steam Flow to Preconditioner (%) 10 8Water Flow to Preconditioner (kg/hr) 125 280 Water Flow toPreconditioner (%) 10.5 14 Weight in Preconditioner (kg) 82 122Preconditioner Retention Time (min) 3.37 2.69 Preconditioner DischargeTemp (° C.) 92 81 Moisture Entering Extruder (% wb) — 25.02 EXTRUSIONINFORMATION S_(D)/R_(D) 1.574 2.351 Size of Extruder Drive (kW) 187.5112.5 Extruder Shaft Speed (RPM) 441 425 Extruder Motor Load (%) 89 26.2Steam Flow to Extruder (kg/hr) 0 280 Steam Flow to Extruder (%) 0 14Water Flow to Extruder (kg/hr) 90 0 Water Flow to Extruder (%) 7.6 0Control/Temperature First Head (° C.) 50/58 — Control/Temperature SecondHead (° C.) 50/61 — Control/Temperature Third Head (° C.) 50/91 —Control/Temperature Fourth Head (° C.)  50/105 — Specific MechanicalEnergy (SME) (kWhr/T) 82.6 9.5 Specific Mechanical Energy (SME) (kJ/kg)297.4 342 Steam Energy (STE) (kJ/kg) 288 619.2 Total Energy Input (SME +STE) 585.4 653 Energy Costs ($/T) 7.48 5.15 FINAL PRODUCT INFORMATIONExtruder Discharge Moisture (% wb) 20.73 24.76 Extruder DischargeDensity (kg/m³) 318 294

These runs confirm that use of the improved extruder permitssignificantly greater steam injection into the barrel as compared withthe prior design. These products were greatly expanded as compared tothe products of Example 1, as evidenced by their lower densities.Nonetheless, the improved extruder gave such lighter products at muchreduced costs.

EXAMPLE 3

In this example cat food products were prepared using the basic extruderassembly of FIGS. 1-5 and the preconditioner of FIGS. 8-9. The objectiveof these tests was to prepare a light density (320-350 g/l, or about20-22 lb/ft³) cat food using less than 10 kWhr/T SME input with 12% byweight steam injection into the extruder barrel. The dry ingredientrecipe was made up of 53% by weight corn, 22% by weight poultry meal,15% by weight soybean meal, and 10% corn gluten meal. This recipeprovided 32.6% by weight protein, 4.0% by weight starch, 34.9% by weightfat, and 2.9% by weight fiber. The results of this run are set forth inTable 4.

TABLE 4 Product Cat Food RUN NUMBER 5 6 7 DRY RECIPE INFORMATION Density(kg/m) 556 556 556 Feed Rate (kg/hr) 2000 2000 2000 Feed Screw Speed(RPM) 40 40 40 PRECONDITIONING INFORMATION Preconditioning Speed (RPM)50/800 50/800 50/800 Steam Flow to Preconditioner (kg/hr) 166 166 166Steam Flow to Preconditioner (%) 8.3 8.3 8.3 Water Flow toPreconditioner (kg/hr) 114 114 114 Water Flow to Preconditioner (%) 5.75.7 5.7 Weight in Preconditioner (kg) 120 120 120 PreconditionerRetention Time (min) 2.64 2.64 2.64 Preconditioner Discharge Temp 83 8383 (° C.) S_(D)/R_(D) 2.351 2.351 2.351 Size of Extruder Drive (kW)112.5 112.5 112.5 Extruder Shaft Speed (RPM) 725 725 425 Extruder MotorLoad (%) 15 18 20-21 Steam Flow to Extruder (kg/hr) 336 236 — Steam Flowto Extruder (%) 16.8 11.8 11.9 Water Flow to Extruder (kg/hr) 0 0 0Water Flow to Extruder (%) 0 0 2.3 Specific Mechanical Energy 8 10 11(SME) (kWhr/T) Specific Mechanical Energy 28.8 36 39.6 (SME) (kJ/kg)FINAL PRODUCT INFORMATION Extruder Discharge Moisture (% wb) 19.62 18.4022.39 Dryer Discharge Density (kg/m³) 303 318 307 % Cook 92.7 94.6 94.7

These tests confirm that high-quality, light-density cat food can bemade at 8-11 kWhr/T SME input. Typical prior art cat foods require anSME input of 25-40 kWhr/T. The cook values obtained were very high andthe moisture levels were somewhat lower than normal for extruded catfeeds.

EXAMPLE 4

In this example floating fish feed products were prepared using thepreconditioner-extruder setup of Example 3. The objectives of thesetests were to determine if a good-quality fish feed could be produced atvery low SME inputs, and whether such feeds could be produced byremoving a substantial part of the added water.

The dry ingredient recipe included 35% by weight wheat midlings, 17% byweight rice bran, 23% by weight wheat flour, 6% by weight fish meal, and19% by weight soybean meal. The dry recipe provided 24.0% by weightprotein, 3.0% by weight fat and 36.0% by weight starch. The results ofthis test are set forth in Table 5.

TABLE 5 Product Floating Fish Feed RUN NUMBER 8 9 DRY RECIPE INFORMATIONDensity (kg/m) 508 508 Feed Rate (kg/hr) 2000 2000 Feed Screw Speed(RPM) 40 40 PRECONDITIONING INFORMATION Preconditioning Speed (RPM)50/800 50/800 Steam Flow to Preconditioner (kg/hr) 154 154 Steam Flow toPreconditioner (%) 7.7 7.7 Water Flow to Preconditioner (kg/hr) 100 100Water Flow to Preconditioner (%) 4.5 4.5 Weight in Preconditioner (kg)120 120 Preconditioner Retention Time (min) 2.64 2.64 PreconditionerDischarge Temp (° C.) 73 73 S_(D)/R_(D) 2.351 2.351 Size of ExtruderDrive (kW) 112.5 112.5 Extruder Shaft Speed (RPM) 720 720 Extruder MotorLoad (%) 23 20 Steam Flow to Extruder (kg/hr) 250 242 Steam Flow toExtruder (%) 4.5 3.8 Water Flow to Extruder (kg/hr) 0 0 SpecificMechanical Energy (SME) (kWhr/T) 12 10 Specific Mechanical Energy (SME)(kJ/kg) 43.2 36 FINAL PRODUCT INFORMATION Extruder Discharge Moisture (%wb) 21.58 22.46 Extruder Discharge Density (kg/m³) 384 422 DryerDischarge Density (kg/m³) 366 392 % Floating Product 100 100

These tests confirmed that good floating fish feeds can be made usingthe extruder and processes of the invention, even with low cost feedingredients such as rice bran and soybean meal. SME values as low as 10kWhr/T were achieved. The extruder was operated at rates as high as fourton per hour while maintaining acceptable product quality. Consequently,only a 60 hp drive is required for such rates. Very importantly, it wasstill possible to inject 12% by weight steam into the extruder barrel atthe four ton per hour dry feed rate, indicating that the longer pitchregions of the screws still provided enough free volume to accept theinjected steam. The as-extruded moisture levels were at lower moisturelevels for conventional feeds.

EXAMPLE 5

In this example a comparative test was run using a conventional TX85Wenger extruder with a standard DDC preconditioner (Run 11), andalternately the preferred extruder/preconditioner of the presentinvention (Run 10). A cat food product of identical formulation was usedin each test, made up of 53% by weight corn, 22% by weight poultry meal,15% by weight soybean meal, and 10% by weight corn gluten meal. The dataderived from these comparative runs is set forth in the following Table6.

TABLE 6 Product Cat Feed RUN NUMBER 10 11 DRY RECIPE INFORMATION Density(kg/m) 621 621 Feed Rate (kg/hr) 2000 2000 Feed Screw Speed (RPM) 36 37PRECONDITIONING INFORMATION Preconditioning Speed (RPM) 50/800 125/250Steam Flow to Preconditioner (kg/hr) 160 120 Steam Flow toPreconditioner (%) 8 6 Water Flow to Preconditioner (kg/hr) 136 280Water Flow to Preconditioner (%) 6.8 14 Weight in Preconditioner (kg)111 119 Preconditioner Retention Time (min) 2.90 3.1 PreconditionerDischarge Temp (° C.) 94 75 S_(D)/R_(D) 2.351 1.574 Size of ExtruderDrive (kW) 112.5 187.5 Extruder Shaft Speed (RPM) 700 726 Extruder MotorLoad (%) 33 65 Steam Flow to Extruder (kg/hr) 273 55 Steam Flow toExtruder (%) 13.6 2.75 Water Flow to Extruder (kg/hr) 0 59 Water Flow toExtruder (%) 0 2.95 Temperature First Head (° C.) — 90 TemperatureSecond Head (° C.) — 139 Temperature Third Head (° C.) — 114 TemperatureFourth Head (° C.) 97 126 Final Head Pressure (psig) 500 250 SpecificMechanical Energy (SME) (kWhr/T) 11.4 62.8 Specific Mechanical Energy(SME) (kJ/kg) 41.04 226 Barrel Retention Time (sec.) 7.25 7.0 FINALPRODUCT INFORMATION Extruder Discharge Moisture (% wb) 22.57 25.33Extrucler Discharge Density (kg/m³) 342 315 Dryer Discharge Density(kg/m³) 338 308 % Cook 93.5 83.6

As will be readily appreciated upon consideration of the data of Table6, use of the preferred preconditioner significantly reduced SME by overa factor of five, while still giving a fully equivalent cat feed asproduced using the conventional equipment. It is also noteworthy thatthe steam flow to the extruder in Run 10 was much increased as comparedwith the conventional equipment.

The comparative products produced in this example were analyzed usingscanning electron micrographs (SEMs) and X-ray tomography. This studydemonstrated that the microstructures of the conventional product andthat produced using the preferred system of the invention were somewhatdifferent. Generally, the product of the invention had somewhat thickercell walls and smaller cells.

The comparative products were also subjected to a palatability feedingtest using a Wilcoxon Signed Rank Test. Twenty male and female catsidentified by ear tattoo and cage number were presented the test dietson an individual basis. Bowl placement was reversed daily and both bowlswere presented for four hours. Two bowls, each containing approximately100 gm of diet were offered once daily for two days. If one diet wascompletely consumed prior to the end of the four hours, both bowls wereremoved. Food consumption and first choice preference were recorded foreach cat. The statistics applied were the Wilcoxon tests to establishnonparametric ranking of observed consumption differences and ananalysis of variance in consumption. Individual t-tests were performedto determine the consumption preference of each cat. A Chi Square testwas performed to establish the significance in first choice preference.The results of this palatability study demonstrated no significancedifferences in palatability between the comparative feed products. Thiswas surprising, inasmuch as high SME inputs are generally necessary forthe production of highly palatable feeds.

1. A screw assembly for use in a twin-screw cooking extruder andcomprising: first and second elongated, axially rotatable screws eachhaving an elongated shaft with outwardly extending helical flightingalong the length of each shaft, the flighting of each shaft beingintermeshed with the flighting of the other shaft, said first and secondscrews being substantially coextensive in length, the flighting of eachof the first and second shafts presenting a pair of axially spaced apartsections of short pitch length, and an intermediate section having apitch length greater than the pitch lengths of either of said shortpitch length sections, said intermediate section having an axial lengthgreater than four times the length of each of said axially spaced apartsections, said screws having an S_(D)/R_(D) ratio of from about 1.9-2.5,wherein S_(D) is the outermost screw diameter of the flighting of saidfirst and second shafts, and R_(D) is the root diameter of said firstand second shafts.
 2. The screw assembly of claim 1, the flighting ofsaid intermeshed intermediate sections having an axial gap distancethere between of from about 0.1-0.4 inches.
 3. The screw assembly ofclaim 2, said gap distance being from about 0.15-0.35 inches.
 4. Thescrew assembly of claim 1, wherein said S_(D)/R_(D) ratio is about 2.35.5. The screw assembly of claim 1, wherein the ratio of the pitch lengthof said intermediate section to the pitch lengths of either of saidfirst and second sections is from about 3-6.
 6. The screw assembly ofclaim 1, said screw flighting oriented for co-rotation of the first andsecond screws.
 7. The screw assembly of claim 1, the flighting of eachof said first and second shafts having equal screw diameters.
 8. Thescrew assembly of claim 1, the root diameters of said first and secondshafts being the same.
 9. The screw assembly of claim 1, the flightingof said first and second shafts being single flight.
 10. The screwassembly of claim 1, wherein the opposed ends of said intermediatesection abut said pair of short pitch length sections.